linux-kernel - Re: [RFC PATCH v6 036/104] KVM: x86/mmu: Explicitly check for MMIO spte in fast page fault

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Date:   Tue, 2 Aug 2022 09:34:02 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "erdemaktas@...gle.com" <erdemaktas@...gle.com>,
        Sagi Shahar <sagis@...gle.com>
Subject: Re: [RFC PATCH v6 036/104] KVM: x86/mmu: Explicitly check for MMIO
 spte in fast page fault

On Mon, Aug 1, 2022 at 6:46 PM Huang, Kai <kai.huang@...el.com> wrote:
>
> > On Mon, Aug 01, 2022, David Matlack wrote:
> > > On Thu, May 05, 2022 at 11:14:30AM -0700, isaku.yamahata@...el.com wrote:
> > > > From: Sean Christopherson <sean.j.christopherson@...el.com>
> > > >
> > > > Explicitly check for an MMIO spte in the fast page fault flow.  TDX
> > > > will use a not-present entry for MMIO sptes, which can be mistaken
> > > > for an access-tracked spte since both have SPTE_SPECIAL_MASK set.
> > > >
> > > > MMIO sptes are handled in handle_mmio_page_fault for non-TDX VMs, so
> > > > this patch does not affect them.  TDX will handle MMIO emulation
> > > > through a hypercall instead.
> > > >
> > > > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> > > > Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> > > > ---
> > > >  arch/x86/kvm/mmu/mmu.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index
> > > > d1c37295bb6e..4a12d862bbb6 100644
> > > > --- a/arch/x86/kvm/mmu/mmu.c
> > > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > > @@ -3184,7 +3184,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu,
> > struct kvm_page_fault *fault)
> > > >           else
> > > >                   sptep = fast_pf_get_last_sptep(vcpu, fault->addr,
> > &spte);
> > > >
> > > > -         if (!is_shadow_present_pte(spte))
> > > > +         if (!is_shadow_present_pte(spte) || is_mmio_spte(spte))
> > >
> > > I wonder if this patch is really necessary. is_shadow_present_pte()
> > > checks if SPTE_MMU_PRESENT_MASK is set (which is bit 11, not
> > > shadow_present_mask). Do TDX VMs set bit 11 in MMIO SPTEs?
> >
> > This patch should be unnecessary, TDX's not-present SPTEs was one of my
> > motivations
> > for adding MMU_PRESENT.   Bit 11 most definitely must not be set for MMIO
> > SPTEs.
>
> As we already discussed, Isaku will drop this patch.

Ah, I missed that discussion. Can you share a link so I can catch up?